This invention relates to an insulating substrate for use as a printed circuit board utilized in various electric devices and electronic devices and a method of manufacturing a printed circuit board utilizing the insulating substrate.
An insulating substrate impregnated with phenolic resin, for example, is generally used as the insulating substrate for preparing a printed circuit board. Such substrate is prepared by impregnating an insulating sheet such as a sheet of craft paper or a glass cloth with a thermosetting resin such as a phenolic resin varnish and then laminating a plurality of resin impregnated insulating sheet (generally called a prepreg), heat-curing the lamination by a hot press, and then cooling the cured lamination under pressure.
Such insulating substrates are now widely used to prepare printed circuit boards, but they have a detrimental disadvantage that their manufacturing cost is high. Yet the insulating substrates have been used because they have a number of characteristics suitable for manufacturing printed circuit boards.
I have found that the high manufacturing cost is caused by the following reasons.
First reason is that, the prepreg is manufactured by the steps of impregnating a sheet of craft paper or a glass cloth with a thermosetting resin, and then drying the impregnated sheet. These steps increase with manufacturing cost because the thermosetting resin is expensive and because impregnation and drying require a long time and trouble.
Another reason is that a number of such expensive prepregs are laminated. More particularly, a prepreg generally has a thickness of 0.2 mm, so that it is necessary to laminate 8 prepregs in order to obtain an insulating substrate having a thickness of 1.6 mm. Accordingly, the cost of the prepregs occupies a substantial percentage of the manufacturing cost of the resulting insulating substrate.
Further, where such insulating substrate is used to prepare a printed circuit board there are the following problems.
For example, to apply solder to the conductors on a printed circuit board, the insulating substrate is heated to a temperature of about 260.degree. C. so that the gas contained in the insulating substrate is caused to expand by the heat of soldering on the impregnated liquid remaining in the substrate evaporates, expands and breaks out between the insulating substrate and the conductors printed thereon, thus causing blistering and deformation of the printed circuit board. In order to avoid this difficulty it is necessary to complete the soldering operation in a short time.
Further, the printed circuit board is often provided with perforations by punching. However, as the prior art insulating substrate is made up of only prepregs it has a large Young's modulus with the result that cracks and fractures are formed at the time of punching. For this reason, usually the punching operation is performed by decreasing the Young's modulus by heating the substrate to a temperature of 60.degree. C. to 150.degree. C.